1. Field of the Invention
The present invention relates to a steering system capable of altering the mode in which steerable wheels follow to the manipulation of a steering handle, in dependence on the traveling state of a vehicle.
2. Discussion of the Related Art
Heretofore, as steering systems of this kind, there have been known for example, an electric power steering device, (refer to Patent Document 1) capable of altering the assist force for the steering handle manipulation in dependence on the vehicle speed and a variable transmission ratio steering device capable of altering the transmission ratio between a steering handle and steerable wheels in dependence on the vehicle speed (refer to Patent Document 2). Further, there has recently been developed a vehicle capable of switching the vehicle drive mode between a two-wheel drive mode and a four-wheel drive mode in dependence on the traveling state (refer to Patent Document 3), and it may be envisaged that the vehicle of this kind may be provided with a steering system like the aforementioned electric power steering device or the aforementioned variable transmission ratio steering device.
[Patent Document 1]: Japanese unexamined, published patent application No. 2001-253357 (paras. [0008] to [0010])
[Patent Document 2]: Japanese Patent No. 3232032 (paras. [0020] and [0024])
[Patent Document 3]: Japanese unexamined, published patent application No. 2003-127690 (paras. [0002] and FIG. 1)
By the way, where the gradient of the road surface on which a vehicle is traveling varies or where the drive mode is altered between the two-wheel drive mode and the four-wheel drive mode, a variation is also made in a friction resistance that the steerable wheels receive from the road surface by being steered. However, in the prior art steering system, steering controls are not performed to cope with changes in the road surface as well as the drive mode. Therefore, where the gradient of the road surface or the drive mode varies, it may be the case that variations take place in the steering resistance the driver receives on a steering handle as well as in the responsiveness of the vehicle turn to the steering manipulation, so that a strange feeling in steering may be given to the driver.